It has been well known that polyethylene terephthalate, polybutylene terephthalate, and other polyesters can be effectively modified by an organopolysiloxane to obtain improvements in characteristics such as the hydrophobic property, water repellent property, weatherability, and lubricating property.
For example, as disclosed in U.S. Pat. Nos. 4,496,704; 4,452,962; and 4,348,510; when a diorganopolysiloxane containing alcoholic hydroxyl groups at its two terminals is added during the polymerization reaction of the polyester, it is possible to obtain a diorganopolysiloxane block-type polyester with improved surface smoothness. However, the diorganopolysiloxane-modified polyester obtained in this way has a disadvantage. In order to improve the surface smoothness and hydrophobic property by incorporating the diorganopolysiloxane portion into the main chain of this linear copolymer, the copolymerization rate of the diorganopolysiloxane must be increased. However, as the copolymerization rate of the diorganopolysiloxane is increased, the characteristics of the polyester disappear.
Furthermore, since the diorganopolysiloxane containing alcoholic hydroxyl groups at its two terminals lacks compatibility with the starting material polyester monomer, copolymerization is difficult and there is a significant residual portion that is not copolymerized.